When is a solenoid valve usually closed in a control system?

A solenoid valve is usually closed when the coil is de-energized. This is because the operation of solenoid valves relies on electromagnetic principles. When electrical current flows through the coil, it creates a magnetic field that causes the valve to open, allowing fluid to pass through. Conversely, when the coil is de-energized (meaning no electrical current is supplied), the magnetic field dissipates, and the valve mechanism returns to its default position, which is typically the closed position.

In control systems, it's essential to understand this behavior because it governs how and when to control fluid flow. The other options refer to different actions that don't directly relate to the inherent function of the solenoid valve's operation regarding its closing state. For instance, a digital input being high typically corresponds to the valve being open, and activating a motor may not influence valve status directly without additional controls. Similarly, increased pressure could lead to different responses depending on the valve's design and system configuration but does not directly indicate that the valve is closed. Thus, understanding the basic relationship between coil energization and solenoid valve position is crucial in control system design and functionality.